A photonic network having an optical add-drop multiplexer and/or a wavelength crossconnect has been proposed and developed. The optical add-drop multiplexer (ROADM: Reconfigurable Optical Add/Drop Multiplexer) is capable of dropping an optical signal of a desired wavelength from a WDM optical signal and guiding the dropped signal to a client, and is capable of adding a client signal of any wavelength to a WDM optical signal. The wavelength crossconnect (WXC: Wavelength Cross Connect or PXC: Photonic Cross Connect) is capable of controlling the route of an optical signal for each wavelength, without converting the optical signal into an electric signal.
In a photonic network as described above, a plurality of optical paths (here, wavelength paths) that use the same wavelength may be set. For this reason, in order to reliably establish and operate a network, for example, a scheme to superimpose a path ID to identify each optical path on an optical signal has been proposed. In this case, an optical node device (here, the optical add-drop multiplexer, the wavelength crossconnect and the like) has a function to detect the path ID superimposed on the optical signal. Accordingly, since each optical path can be identified with certainty at the optical node device, it becomes possible to monitor/detect/avoid a failure such as connecting the optical fiber to a wrong port, and so on.
For example, Japanese Laid-open Patent Publication No. 2013-9238 and Japanese Laid-open Patent Publication No. 2014-150447 disclose a configuration and a method for detecting a signal superimposed on an optical signal.
In a conventional WDM transmission system, wavelength channels are often arranged with a 100 GHz spacing or a 50 GHz spacing. These days, in order to increase a communication capacity, a method for decreasing a spacing with which wavelength channels are arranged is proposed. For example, a method for generating an optical signal of each wavelength channel by use of a Nyquist filter is proposed.
However, when a spacing with which wavelength channels are arranged is decreased, it is difficult to detect a signal superimposed on an optical signal of each wavelength channel. In other words, there is a possibility that a signal superimposed on an optical signal of a target wavelength channel will not be detected due to a signal being superimposed on an optical signal of an adjacent wavelength channel. Further, there is a possibility that a detection circuit will erroneously detect the signal superimposed on the optical signal of the wavelength channel that is adjacent to the target wavelength channel.